Improvents to PBS class and machine.py

batchq/batch/commandline.py:
- PBS._get_pbs_as_file:
  Removed _pushw call since it should only be called once
  Removed _popw call: not needed anymore
- PBS.get_job_info: *new
  Return complete provenance information for the job
- PBS.generate_identifier: *new
  Adds working directory to the job identifier hash

batchq/core/stack.py:
- use_machine: *new
  Enables selecting machine without re-checking ~/.batchq/bin files

batchq/pipelines/shell/sftp.py:
- SFTPTerminal.pwd: Set self._lazy_pwd